This invention relates to a process for dry chlorination of powdery vinyl chloride polymers, and more particularly vinyl chloride homopolymers, with gaseous chlorine.
As used herein, the term "vinyl chloride polymers" is understood to mean homopolymers of vinyl chloride and copolymers of vinyl chloride with at most 20 mol percent of one or more other monomers. Aside from their many excellent properties, vinyl chloride polymers have the considerable drawback in that they become unusable at only slightly elevated temperatures of approximately 60.degree. to 70.degree. C. and lose their dimensional stability. Because of this disadvantage, vinyl chloride polymers are not used as such in major applications like tubes or vessels for hot liquids.
It is known that vinyl chloride polymers can be provided a greater heat resistance by chlorination. The Vicat temperature, that is the softening temperature of the polymer under a load of 5 kg/cm.sup.2, as accurately described in the standard ISO-R 306, corresponding to ASTM-D 1525, is generally used as a measure of the dimensional stability at elevated temperatures.
Chlorinated vinyl chloride polymers not only have a higher Vicat temperature, but also possess better fireresistant properties and have a higher resistance against chemicals than the original (unchlorinated) vinyl chloride polymers, while at the same time the linear expansion coefficient remains low. On the other hand, afterchlorinated vinyl chloride polymers are more difficult to process and more brittle. However, these disadvantages can be completely or substantially eliminated by mixing with non-afterchlorinated vinyl chloride polymers and/or flow and impact strength improving agents.
In practice, a number of processes have become known for chlorination of vinyl chloride polymers, particularly vinyl chloride homopolymers. For instance, polyvinyl chloride can be chlorinated in solution. Owing to the poor solubility of vinyl chloride polymers, such chlorinations are conducted at temperatures above 100.degree. C., as a rule at 120.degree.-130.degree. C., in inert solvents like chlorobenzene or perchlorethylene. Such processes are expensive and do not yield a powdery product, and for that reason are only used if the price of the ultimate product is less critical, as in compounding afterchlorinated vinyl chloride polymers for use in lacquers, glues, lining preparations, and the like.
Powdery (particulate) vinyl chloride polymers, preferably the homopolymers are used in the building industry for hot water pipes, sewer pipes, tubes for central heating, colored window sections, for plate sections and in the chemical processing industry for vessels and lines for storage and transport of hot or corrosive liquids, reactors for the electro-chemical industry and the like. If chlorinated vinyl chloride polymers are used as the fabricating resin, these, too, are desirable in powder form which is well suited for stabilization and further processing according to current compounding techniques, and to achieve this end, chlorination processes are used in which the chlorinated vinyl chloride polymer products are obtained in powder form. This is in contrast to the solution chlorination process described above.
Powdery chlorinated vinyl chloride polymers prepared by aqueous suspension processes are described in several publications or patents such as British Pat. specifications Nos. 1,081,057, 948,372, 976,001, 1,120,769 and 1,186,847, German Pat. Application No. 1,811,472, U.S. Pat. Nos. 3,632,848 and 3,345,140 and French Pat. Nos. 1,286,811, 1,309,937 and 1,580,070. According to the general procedures there described, powdery polyvinyl chloride is suspended in water, then, in the presence of a radical initiator, chlorine is passed through the PVC suspension at elevated temperatures until a vinyl chloride polymer is obtained the required pre-determined chlorine content.
A drawback of the aqueous suspension process is that chlorine dissolves only poorly in water at elevated temperatures and even at increased pressures the chlorination still proceeds slowly. For instance, at a pressure of 4 kg/cm.sup.2 and at temperatures rising form 35.degree. to 85.degree. C., with lauroyl peroxide of di-isopropyl peroxide dicarbonate as radical initiator, a reaction time of 12.5 hours is needed to increase the chlorine content, from 56.8% by weight for normal polyvinyl chloride to 67.9% by weight for the more chlorinated product.
Another process of forming chlorinated vinyl chloride polymers is the gel-phase process. The drawback of the long reaction times required for the aqueous-suspension processes are not present in the gel phase processes. The gel phase process is based on the fact that vinyl chloride polymers and chlorinated vinyl chloride polymers are insoluble below 80.degree. C. in carbon tetrachloride and chloroform, but swell in such solvents to become gel-shaped products.
In illustrative gel phase processes the powdery vinyl chloride polymer is suspended in one of the solvents mentioned, whereupon, with stirring and in the presence of a radical initiator such as a peroxide compound or a radical-initiating radiation, at 50.degree.-60.degree. C. chlorine is passed through the suspension. Processes based on this procedure are described in U.S. Pat. No. 3,627,853, British Pat. Specification No. 1,325,579 German Pat. No. 1,208,889, and Italian Pat. No. 852,492. The vinyl chloride polymer particles swell substantially in the carbon tetrachloride or the chloroform solvents and diffusion of chlorine- and initiator-molecules into such highly swollen particles proceeds well. Chlorine is very soluble in the diluent and, hence, relatively high chlorination speeds can be reached requiring only 2 to 21/2 hours for preparation of a chlorinated polyvinyl chloride polymer having a chlorine content of up to about 68% by weight.
As a result of the substantial swelling only rather dilute suspensions of at most about 20% by weight of vinyl chloride polymer can be used as a starting suspension, since the suspensions would otherwise no longer be stirrable. A disadvantage of these processes is that the swelling increases as the chlorine content increases and that above a chlorine content of about 68% by weight, rapid disintegration of the particles occurs, at which point stirring is for a practical matter no longer possible. In that case a powdery product cannot be recovered from the viscous gel mass when this is processed, unless costly grinding treatments are applied.
In the early sixties an aqueous gel phase process was invented which is described in U.S. Pat. No. 2,996,489, and using this process a commercial scale chlorination procedure is available. In the aqueous gel phase process chlorination takes place in a stirred three-phase system, which phases are polymer particles, carbon tetrachloride or chloroform and water. By using water as a phase in the system, less carbon tetrachloride or chloroform will suffice, the system is stirred easier, while reasonably high chlorination speeds can still be attained. In connection with the disintegration of the polymer particles, in this process chlorination can also be completed to a chlorine content of at most about 68% by weight.
Finally, processes have also been described in which vinyl chloride polymers, in a fluidized state or stirred mechanically in one way or other, are chlorinated with gaseous chlorine; see British Pat. Specifications Nos. 1,089,323, 1,242,158, and 1,318,078, U.S. Pat. Nos. 3,532,612, 3,663,392 and 3,813,370, and German Pat. Nos. 1,110,873 and 1,259,573.
According to these procedures the chlorination is conducted at temperatures of between 40.degree. and 140.degree. C. in the presence of radical-initiating radiation or of an initiator which forms gaseous or solid radicals. Generally, these processes are not conducted isothermally. A moderately elevated temperature is the starting point, the temperature being raised gradually during the course of the chlorination usually to values at which radical formation by thermal initiation also starts to play a part in the chlorination procedure. The direct use of temperatures at which thermal initiation plays a part causes disintegration of the vinyl chloride polymer which has not yet been converted. However, a reproducible manner of practicing an exothermic gas phase process, in which the temperature is to be varied under controlled conditions, is extremely difficult to conduct. A process of this kind, further, cannot be carried out continuously; for a continuous process one should either chlorinate at one definite temperature, or conduct the chlorination in two or more steps.
It has been observed that a vinyl chloride polymer chlorinated according to a gas phase process, or an aqueous-suspension process, has an appreciably lower Vicat temperature than a vinyl chloride polymer with the same chlorine content but having been afterchlorinated according to a gel phase process. According to the U.S. Pat. No. 2,996,489 the ratio between 1,2-dichloroethane units and 1,1-dichloroethane units, which are formed in the after-chlorination, have a considerable influence on this, however subsequent investigations by Trautvetter (Kunststoffe Plastics 2 (1966) 54-58) have proven this view to be incorrect. It is now assumed that in the gas phase processes an inhomogeneous chlorination of the vinyl chloride polymer particles is the cause for the relatively low Vicat temperature.
In the gas phase chlorination the vinyl chloride polymer particles are not swollen and, hence, except at the surface, are difficult for the access to both chlorine molecules and initiator molecules. If radical-initiating radiation is used rather than an initiator it does not penetrate, or if it does, penetrates in a greatly weakened state into the interior part of the particles. Solid, powdery initiators cannot diffuse into the polymer particles. Chlorine radicals formed in the gas phase can penetrate into the particles only after the external layers are completely chlorinated. Only gaseous initiators could diffuse in the polymer particles and thus eliminate the objections regarding the inhomogeneous chlorination. The use of fluorine as a gaseous initiator is described in U.S. Pat. No. 3,813,370.
Efforts to use thermal radical initiation have not been successful. Thermal initiation only becomes significant as contributing to the processing at temperatures of at least 75.degree. C. and reasonable chlorination speeds are only obtained at a temperature of at least 100.degree. C. At temperatures above about 80.degree. C., however, thermal degradation of vinyl chloride polymers occurs. For this reason gas phase processes are as a rule carried out in their entirety below 80.degree. C. and in the presence of an initiator, or in a 2-step process partly below 80.degree. C. with a radical-forming initiator or radiation, and then partly at higher temperatures. Chlorinated polyvinyl chloride has a better thermal stability than the polyvinyl chloride itself and it appears that after a certain period of chlorination, the temperature may be increased without any risk of the vinyl chloride polymer being disintegrated. A difficulty involved in this process is that one cannot establish accurately when the temperature of the reaction may safely be raised without disintegration occurring. Of course, the major objection remains that the Vicat temperatures are considerably lower than is the case in the chlorination according to the gel phase processes.